This invention relates in general to vehicle stability control systems and in particular to a method to compensate for mounting errors of the inertial sensors used in vehicle stability control systems.
Performing vehicle stability control (VSC) with an electronic control unit (ECU) requires accurate inputs of the vehicle's inertial state. This is accomplished with a sensor for detecting and measuring vehicle rotation about a vertical axis, that is, a yaw rate sensor, and either a single lateral accelerometer or a lateral accelerometer and a longitudinal accelerometer. Regardless of how many accelerometers are utilized, it is necessary that all of the internal state sensors are accurately mounted in the vehicle. Ideally, the rotational velocity sensors and accelerometers are mounted with their principle sensing axis aligned with the vehicle's three spatial axes. The three vehicle spatial axes for a vehicle 10 are illustrated in FIG. 1 where the arrow labeled 12 corresponds to the vehicle longitudinal axis that runs along the length of the vehicle and points toward the forward direction of movement for the vehicle. A second spatial axis labeled 14 corresponds to the vehicle lateral axis and is perpendicular to the longitudinal axis 12. A third spatial axis labeled 16 that extends perpendicular to the plane formed by the longitudinal and lateral axes 12 and 14 corresponds to a vertical axis of the vehicle. Thus, an accelerometer for measuring acceleration and velocity of the vehicle would be ideally mounted with its sensing axis parallel to the vehicle longitudinal axis 12 while an accelerometer for measuring lateral motion of the vehicle would be mounted with its sensing axis parallel to vehicle lateral axis 14.
Three spatial rotational velocities are also illustrated in FIG. 1 where the circular arrow labeled 18 that is centered upon the longitudinal axis 12 corresponds to vehicle roll velocity while the circular arrow labeled 20 that is centered upon lateral axis 14 corresponds to vehicle pitch velocity. Similarly, the circular arrow labeled 22 that is centered upon the vertical axis 16 corresponds to yaw velocity. Each of these rotational velocities may be measured by a rotational velocity sensor that would ideally have its axis of rotation parallel to the vehicle spatial axis about which the rotational velocity occurs. Thus, for a measurement of yaw velocity, the corresponding yaw velocity sensor would have its axis aligned with the vehicle vertical spatial axis 16.
If the yaw rate sensor and accelerometers are not accurately mounted, erroneous information will be transferred to the ECU. Signal error due to mounting, which is the difference between the actual inertial state and the sensor's measured state, is a function of cos ⊖, where ⊖ is the angle between the axis of measurement and the actual sensor mounting. For example, a sensor 24 that is intended to measure yaw rate is illustrated in FIG. 2 where the spatial axes of the vehicle are shown by the solid arrows labeled 12, 14 and 15. In FIG. 2, the vertical axis of the sensor 24 is aligned with the vehicle vertical axis 16; however, the lateral axis of the sensor 24, that is illustrated by the dashed arrow labeled 26 is offset by the angle ⊖ from the vehicle pitch axis 14. For small ⊖, this error is not significant; however, cross-axis error, which is sensitivity to inertia in an axis not intended to be measured, is a function of sin ⊖. For small ⊖, the cross-axis error is significant. For example, consider that the sensor 24 shown in FIG. 2 has a 5° mounting error relative to the pitch axis 14. During an actual 50°/sec yaw maneuver, the sensor would have an output of (cos 5°)*50°/sec, or 49.8°/sec. However, during 100°/sec pitch maneuver, which has a zero °/sec yaw, the yaw sensor would have an output of (sin 5°)*100°/sec, or 8.7°/sec instead of zero °/sec. This signal is incremental to any existing errors. Similar errors also would occur for accelerometer measurements.
To prevent unintended interventions by the VSC ECU in response to the errors described above, activation thresholds for the VSC are increased. However, increasing the thresholds may reduce the sensitivity of the ECU. Accordingly, it would be desirable to minimize the effect of such errors.